Pro-Russia supporters attack a police bus near the regional government building in Kharkiv, Ukraine, on April 7. / Anastasia Vlasova, epa

by Oren Dorell and Anna Arutunyan, USA TODAY

by Oren Dorell and Anna Arutunyan, USA TODAY

MOSCOW - Secretary of State John Kerry on Tuesday accused Russia of paying people to cause unrest in east Ukraine and create a pretext for invasion as Ukraine forced out protesters from government buildings.

"What we see from Russia is an illegal and illegitimate effort to destabilize a sovereign state and create a contrived crisis with paid operatives across an international boundary," Kerry told the Senate Foreign Relations Committee in Washington.

Kerry said the White House is prepared to impose more economic sanctions than have already been enacted if Russia keeps backing upheaval in Ukraine. The White House said Kerry will meet with European, Ukrainian and Russian diplomats in the coming days to try to solve the crisis.

Ukrainian security forces Tuesday forced pro-Russia protesters out of the regional government headquarters in Kharkiv, close to the border with Russia. About 70 protesters who say they want to secede from Ukraine set the building on fire before being arrested. Demonstrators in Donetsk continued to occupy a government building there.

The Russian Foreign Ministry on Tuesday warned Ukraine to stop its military preparations in eastern Ukraine or risk civil war, according to Russian state-owned ITAR-TASS news agency. Ukraine has been trying to shore up its defenses in case of a Russian attack.

NATO warned Russia not to encroach further into the region â?? a move it said would represent a "serious escalation" of the crisis.

"We call on Russia to pull back the tens of thousands of troops it has massed on Ukraine's borders, engage in a genuine dialogue with the Ukrainian authorities and respect its international commitments," NATO Secretary General Anders Fogh Rasmussen in Paris.

Kerry called the demonstrations a "contrived pretext for military intervention just as we saw in Crimea." Last month, Russia invaded the Ukraine province of Crimea and annexed it following protests and a referendum there in which a majority of people allegedly voted to join Russia. The West says the vote and annexation is illegal.

The events in east Ukraine appear to be following the same playbook as Crimea.

Pro-Russia demonstrators in Ukraine's east are getting support from Russians inside Ukraine, though the "volunteers" as they are called may not all be acting on orders from Moscow, say experts and pro-Russia groups.

"They don't make up a big share of the demonstrators, but there are up to a thousand Russian volunteers in Ukraine," said analyst Sergei Markov, a backer of the Russian government who has advised the Kremlin on Ukraine. "They make up maybe 1% demonstrators."

Asked if those volunteers would be willing to take up arms if a conflict broke out, Markov said "of course."

Markov said the actions of the Russian volunteers is "largely" spontaneous. If violent clashes erupt, "the number of volunteers will grow significantly."

"Sometimes they coordinate their activities with (Russian authorities), sometimes they don't," he said.

Thousands of Russian soldiers are based close to the Ukraine border despite assurances made by Russian President Vladimir Putin to European leaders that he was pulling some back.

In March, Alexander Zaldostanov, the head of a biker group called Night Wolves with close ties to Putin, was spotted on a flight from Crimea's capital of Simferopol to Moscow after a referendum in which a majority of Crimeans voted for joining Russia.

Asked what was going to happen next, Zaldostanov said, "we have to keep fighting."

The Eurasian Union of Youth, a patriotic movement in Russia that has existed since 2005, is just one group that has sent volunteers to support Russian speakers in Ukraine, according to Pavel Kanishchev, who heads the union's federal network.

"We have coordinators in east Ukrainian cities, many of them are locals," Kanishchev said.

Russia has refused to deal with Ukraine's new government that ousted pro-Moscow President Viktor Yanukovych. NATO says Russia has tens of thousands of troops along its border with Ukraine, but the EU-US military alliance has not made moves to confront the troops.